It has been desired to develop a modulated light source that is compact and consumes a low power. In such a modulated light source, using a minute ring modulator with a silicon sub-micron optical waveguide has been studied.
FIG. 8 is a diagrammatic view depicting a schematic configuration of a modulated light source of related art using a ring modulator.
The modulated light source includes a distributed feedback (DFB) laser 101, a ring modulator 102, a PD 103, a wavelength controller 104, and a heater 105.
The PD 103 senses power of light having passed through the ring modulator 102. The wavelength controller 104 outputs a signal that controls the wavelength of the ring resonance based on the optical power sensed with the PD 103. The heater 105 heats the ring modulator 102 in accordance with the control signal from the wavelength controller 104 to adjust the wavelength of the ring modulator to match the laser wavelength.
In the modulated light source, the DFB laser 101 outputs laser light in a continuous emission mode. The outputted laser light passes through an optical waveguide and then is guided to the ring modulator 102, which modulates the transmissivity at the laser light wavelength transmissivity.
The ring modulator 102 has a Lorentzian spectrum centered at a resonance wavelength. The ring modulator 102 changes the resonance wavelength in accordance with a change in a modulation signal between voltages V0 and V1. The transmissivity is thus modulated, whereby intensity-modulated output light is produced.
The resonance wavelength of the ring modulator 102 changes as a circumference optical path length of the ring modulator 102 changes due to a manufacturing error and/or a temperature change, resulting in a discrepancy between the resonance wavelength and the wavelength of the laser light being emitted. As depicted in FIG. 9, to compensate for the discrepancy, the heater 105 heats the ring modulator 102 to raise the temperature of it for adjustment of the resonance wavelength.
In this case, however, it is undesirably difficult not only to ensure reliability of the modulated light source but also to improve power efficiency in the modulation or the like (decrease in electric power necessary for heater and modulation operation). The reason for this is as follows.
The case where the ring modulator has a small radius as depicted in FIG. 10A will be considered. In this case, the volume of the ring modulator is small, whereby electric power consumed by the heater that is required to compensate for the wavelength shift resulting from variation in temperature is decreased. Furthermore, the small radius of the ring modulator reduces a capacitance that serves as a load on a drive circuit of the ring modulator, whereby the modulation power is deceased. On the other hand, because the difference between the laser and the ring modulator wavelength amounts up to the free spectral range (FSR), an increased FSR increases the amount of required wavelength compensation, resulting in an increase in the amount of increase in the temperature of the ring modulator and hence a decrease in reliability of the ring modulator.
The case where the ring modulator 102 has a large radius as depicted in FIG. 10B will be considered. In this case, the FSR is small, resulting in a decrease in the amount of wavelength compensation, which reduces the amount of increase in the temperature of the ring modulator, whereby reliability of the ring modulator is ensured. On the other hand, the volume of the ring modulator increases, resulting in increases in modulation power, and electric power consumed by the heater that is required to compensate for the wavelength shift due to variation in temperature.
Furthermore, a problem caused by use of the DFB laser 101 is not negligible. That is, DFB lasers without a phase shift in the diffraction grating for improvement of power efficiency reduces a yield thereof. Conversely, introduction of a phase shift for improvement in the yield lowers the power efficiency.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-64862
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-59729